The present invention relates to a method for the quantitative determination of one or more chemical parameters of a sample medium wherein two substances are used that are closely adjacent to each other, i.e., a first substance in the form of a fluorescent agent with an excitation spectrum and an emission spectrum, this first substance not being responsive to the parameter to be determined, and a second substance in the form of an agent whose absorption spectrum is changed by the parameter of the sample medium to be determined, the emission spectrum of the fluorescent substance at least partially overlapping the absorption spectrum of the second substance.